1. Field of the Invention
The present invention relates to a switching power supply device and an electronic apparatus including the same.
2. Description of the Related Art
In recent years, the demand for minimizing power consumption during standby when a printing operation is not performed in a printer or a facsimile machine, for example, has been increasing. Accordingly, a switching power supply device used for a power supply circuit unit of a printer or a facsimile has been required to reduce power consumption in a standby status.
Typically, a switching power supply device of an RCC (rising choke converter) type is used for a power supply circuit unit of those electronic apparatuses. However, the switching power supply device of the RCC type has a characteristic that a switching frequency increases as a load becomes lighter and the switching loss increases accordingly. Under these circumstances, reduction of the power consumption under a light load, such as in a standby state, cannot be expected.
Japanese Unexamined Patent Application Publication No. 7-7335 discloses an example of a switching power supply device of the RCC type that is capable of reducing power consumption under a light load. This switching power supply device includes a circuit to force a control terminal of a first switching element to be grounded for a predetermined time period under the light load, so as to delay turn-on of the first switching element and to prevent the switching frequency from exceeding a predetermined value.
However, in the switching power supply device in which the upper limit of the switching frequency is set by the above-described circuit, a sufficient effect of significantly reducing power consumption by decreasing the switching frequency under a light load cannot be obtained.
On the other hand, a switching power supply device for overcoming the above-described problem is disclosed in Japanese Unexamined Patent Application Publication No. 2004-80941. The switching power supply device according to this document includes a circuit for setting a minimum on-period in an on-period of a first switching element so that the minimum on-period is ensured. In this case, the on-period cannot be shortened and thus power is excessively supplied to a load in a standby status, so that an output voltage starts to rise. By providing a circuit for controlling (extending) an off-period upon detecting a slight increase in the output voltage, an increase in the output voltage is prevented and an increase in the switching frequency is minimized.
In the switching power supply device according to Japanese Unexamined Patent Application Publication No. 2004-80941, a standby status is detected by detecting that the output voltage has increased. Thus, a difference is caused between a standby output voltage and a rated output voltage. In other words, a variation width of the output voltage is disadvantageously large. Further, two systems of feedback circuits are required: a feedback circuit for controlling an on-period under a non-light load and a feedback circuit for controlling an off-period under a light load. Therefore, a gain changes at switching between those systems and also the output voltage changes at switching between the control systems when the load changes.